There is a considerable growth in the use of oxygen in many industrial fields, especially in the papermaking field, and in particular in kraft-type paper pulp mills.
This is because not only is the size of plants increasing but applications for oxygen are becoming more numerous; in the papermaking field, mention may be made, for example, of the use of oxygen for delignification, the alkaline extraction stage, the doping of lime kilns, the treatment of effluents, the oxidation of liquors and, more recently, the production of ozone.
Since the requirements for oxygen are growing, it is becoming increasingly common, especially for economic reasons, to produce oxygen on the actual site where it is used, making use to do this of self-production systems, for example of the PSA (Pressure Swing Adsorption) or VSA (Vacuum Swing Adsorption) type. Both these types of process consist in making air pass over a column of adsorbents, for example of the zeolite type. Nitrogen is preferably fixed by the adsorbent and an oxygen-rich gas can thus be recovered at the column exit; it is then possible to regenerate the adsorbent by applying a slight reduced pressure in the column. Using a minimum of two columns in parallel with adsorption and desorption cycle phases, continuous delivery of an oxygenated gas rich in oxygen is possible.
Such plants deliver a sufficiently pure oxygenated gas, generally containing 90 to 98% oxygen, compatible with most conventional applications in industry, in particular with most conventional applications in the papermaking industry. A number of processes require only a few adjustments or modifications, especially a simple increase in the gas flow rate, when this type of oxygenated gas is used; the presence of gaseous impurities, particularly nitrogen and argon (inert non-oxidizing gases) does not in fact disturb these processes.
However, this does not necessarily apply in oxidation processes whose operating principle relies on an almost complete consumption of the oxidation gas fed into a reactor working at constant pressure and in which processes the presence of inert gases reduces the oxygen partial pressure in the gas in proportions which depend on the percentage of the said inert gases, also called inert components, in the gas used to carry out the oxidation. This reduction in the oxygen partial pressure impairs the progress of the reaction and makes it necessary to purge the gas overhead. The term “gas overhead” is used both to denote the space lying above the liquid phase in the reactor and to denote the gas contained in this space (the context will allow a person skilled in the art to know without any ambiguity the meaning to ascribe to this term when the distinction proves necessary). When the oxidation gas is pure oxygen, it is possible to maintain this partial pressure and therefore to carry out the oxidation of the substance to be oxidized, contained in the liquid phase. When this gas is not pure oxygen, the same does not apply and the gas overhead becomes enriched with inert components, therefore reducing the oxidation rate.
In general, in the prior art, when an oxidation gas containing inert components is used to oxidize substances contained in a liquid medium by transferring this gas into the said medium, provision is made to purge the gas overhead.
Thus, a conventional stirred tank reactor (or STR) is described in the article by M. Lawrence and M. Litz in CEP, November 1985, pp. 36-39, entitled: “A Novel Gas-Liquid Stirred Tank Reactor”). Within such a stirred tank reactor, the gases are distributed generally by a pierced torus placed at the bottom of the reactor, below the mechanical stirrer, designed, in principle, to disperse the gas throughout the reaction mixture. This type of reactor, allowing high transfer coefficients and thus efficient gas-liquid mixing, provides a simple venting should a diluting gas be present.
Also known, from WO-A-99/04088, is a process for the oxidation of white or black liquors carried out under pressure, at high temperature, in a stirred tank reactor fitted with a multibladed stirrer allowing axial and radial gas/liquid mixing. Located downstream of the said reactor is a gas/liquid separator designed to separate the undissolved gases from the oxidized liquor.
Also known, from WO-A-96/13463, is a process for the oxidation of effluents in an unconventional reactor in the presence of a heterogeneous catalyst, which works in the gas phase provided above the liquid phase. The liquid is stirred by means of an external pump in a recirculation loop. An in-line mixer placed in the recirculation loop intimately mixes gas with the liquid phase. The gas phase is withdrawn via a line.
Moreover, reactors are known, from U.S. Pat. No. 4,328,175 and U.S. Pat. No. 4,454,077, which are equipped with downflow gas/liquid mixing means, consisting of a helical impeller which creates a vortex on the surface of the liquid phase. The gas is injected into the gas overhead in the reactor and, mixed with the liquid by the vortex effect, is entrained into the liquid phase. The pumping flow created by the impeller is used to disperse the gas/liquid mixture throughout the entire volume of the reactor. These reactors are fitted with purging means.
However, none of the processes of the prior art has proposed a method of purging specifically adapted to the oxidation process carried out. At the present time, no simple means is known for purging the gas overhead of a reactor in a controlled manner so as to guarantee the desired oxidation and oxygen-consumption performance throughout the process involved. Insufficient purging of the gas overhead, resulting in an excessive reduction in the oxygen partial pressure, runs the risk of slowing down, or even stopping, the oxidation reaction carried out; excessive purging of the gas overhead certainly removes a larger amount of inert gases, thus admittedly maintaining a certain oxidation, but it results in excessive oxygen consumption and consequently adversely affects the economics of the process.